Access 2007

I have a mainform which contains a subform

The mainform contains details of the finished item
the subform contains details of the finished items parts

Eg
Main form = Table  = sum of 4 x 10 + 1 x 140 = £198
Subform = 4 x legs  @ £10ea + 10% markup
               1 x top @ £140 ea + 10% markup

I would like the mainform to have a running total of how much the item is
costing as new lines are added to the subform.

Subform contains
Itemdesc
ItemQty
ItemUnitRate
ItemMarkUp%
ItemSaleValue - this is a calculated field ......
=(ItemQty*ItemUnitRate)+(ItemQty*ItemUnitRate*ItemMarkUp%)

Mainform contains
FinItemDesc
FinItemQty
FinItemSalValue  - it is this field that I cannot calculate.   I want it to
equal  Sum(ItemSalValue) * FinItemQty

I have tried adding a calculated field in the footer (txtItemTotal) of the
subform as:  sum(ItemSaleValue)
and a calculated field in the mainform as:  forms!frmSubform.txtItemTotal x
FinItemQty

but this doesn't work.

What am I doing wrong

Thanks

A